The present invention relates generally to AGC circuits for television receivers and particularly concerns an AGC gate signal generating circuit for use with a gated AGC detector.
Gated AGC systems for television receivers typically include an AGC detector which is gated for sampling the horizontal sync tip levels of a received video signal. The sampled video level is normally stored on a capacitor and coupled therefrom for adjusting the gain of the RF and IF amplifiers of the receiver to maintain the detected video signal at a substantially constant level.
Early gated AGC systems employed the horizontal flyback pulses developed in the receiver as the gating signal. These systems have the disadvantage that when the receiver is initially turned on phase coincidence between the flyback pulses and the horizontal sync pulses is not always immediately established. As a result, the AGC detector is not gated for sampling the horizontal sync tip level as desired but rather is gated some time during the horizontal scanning interval such that the video signal itself is sampled leading to the development of an incorrect AGC voltage across the storage capacitor.
More recent gated AGC systems have overcome the foregoing problem by ANDing the flyback pulses with the horizontal sync pulses to form the gating signal. However, when a reduced level video signal is received, due to airplane flutter or other interference, the horizontal sync pulses may decrease below their detection level so that no gating signal is produced resulting in a so-called "lockout" of the AGC system. To minimize the effects of AGC lockout, anti-lockout circuits have been developed which sense a shift in the level of the horizontal sync pulses for producing a DC voltage representing the receipt of a weak sync signal. This DC voltage is then processed with the flyback pulses to produce the gate signal for allowing operation of the AGC system. While such circuits provide acceptable performance, they are quite complex and relatively expensive to manufacture.
It is therefore a basic object of the present invention to provide an improved AGC gate signal generating circuit.
It is a more specific object of the invention to provide an improved AGC gate signal generating circuit which is relatively inexpensive to manufacture yet provides satisfactory performance for both nominal and reduced level video signals.